1. Field of the Invention
The present invention relates to a planographic printing plate precursor and a method for producing a planographic printing plate. More particularly, the present invention relates to a planographic printing plate precursor which can be produced by scanning exposure based on digital signals, and a simple method for producing a planographic printing plate using the same.
2. Description of Related Art
A conventional planographic printing plate has been produced by exposing a planographic printing plate precursor to a light through a lith film, then removing non-image portions by dissolving them in a developing solution. Recently, digitalizing technologies have been spread widely in which image information is electronically processed, stored, and output using a computer, and various new image output methods making use of such digitizing technologies have been put into actual use. As a result, a computer-to-plate technology in which active radioactive light having a high directivity such as a laser light is scanned according to image information in the form of digitized data and printing plate is directly produced not via a lith film is eagerly desired, and it is an important technological problem to obtain a printing plate precursor suitable for this. On the other hand, in conventional production processes for a planographic printing plate, a process wherein removing non-image portions by dissolving them after exposure is indispensable. However, since the developing waste solution thereof is alkaline, a method for producing a printing plate which does not require such a wet treatment is eagerly awaited in today's industrial world where great importance is attached to protecting the environment. Thus, because of the already developed technologies for digitalizing image information and the necessity of environmental protection, planographic printing plate precursors which do not require wet treatment and can be produced in dry mode are keenly sought.
Japanese Patent Application Laid-Open (JP-A) Nos. 5-77574, 4-125189, U.S. Pat. No. 5,187,047 and JP-A No. 62-195646 and the like disclose that after image formation, a film produced by sulfonation of polyolefins is used as a planographic printing plate precursor which does not require wet developing treatment, and hydrophilicity of the surface thereof is modified by thermal writing to create a plate precursor material which does not require developing treatment. In this system, an image is formed by de-sulfonating sulfone groups existing on the surface of photosensitive materials by thermal writing, therefore, developing treatment is not necessary, however, there is the drawback that noxious gas is generated in the writing.
U.S. Pat. Nos. 5,102,771 and 5,225,316 suggest a sensitive material obtained by combining a polymer having an acid-sensitive group in the side chain and a photo acid generator, and propose a non-developing system. This plate precursor has the drawbacks that the hydrophilicity thereof is low, it is easily contaminated, and the durability of the plate precursor and clearness of the printed image are inferior, since the acid generated is a carboxylic acid.
As image forming materials having radiation sensitivity suitable for the production of positive non-treated planographic printing plates, those described in JP-A No. 7-186562 are known using specific carboxylates or sulfonates as image forming materials. By using the image forming materials described in this publication, there can be obtained a planographic printing plate which can be developed with water giving a satisfactory print, however, if the energy in exposure is low, there is a tendency that the image forming material near the substrate does not become completely hydrophilic and the image forming material can not be removed completely, and accordingly, the resulting print is blemished.
Further, as a method for producing a printing plate by scanning exposure, a method has been suggested utilizing active radioactive light having a high power density. In general, the recording mode by high power density exposure is called heat mode recording. The reason for this is that in a high power density exposure system, it is believed that photo energy absorbed by a sensitive material is often converted into heat, and the desired phenomenon is caused by the heat generated. A large part of the merit in the heat mode recording method resides in the potential possibility of simple treatment, dry treatment, and no-treatment. This is based on the fact that the phenomenon utilized for image recording of a heat mode sensitive material does not substantially occur by exposure to a light of normal strength or under normal environmental temperatures thereby negating the necessity for image fixing after exposure.
As one preferable method of producing a planographic printing plate based on the heat mode recording, there is a method in which a hydrophobic image forming layer is provided on a hydrophilic substrate, the layer is subjected to image-wise heat mode exposure to alter the solubility and dispersibility of the hydrophobic layer, and where necessary, non-image portions are removed by wet development. As an example thereof, Japanese Patent Application Publication (JP-B) No. 46-27919 discloses a method in which a plate precursor comprising a hydrophilic substrate carrying thereon a recording layer which manifests improved solubility (a so-called positive), action by the effect of heat, and specifically a recording layer having a specific composition of saccharides, melamine-formaldehyde resin and the like, is subjected to heat mode recording to obtain a printing plate. However, heat mode scanning exposure sensitivity has been insufficient since none of the disclosed recording layers has satisfactory heat sensitivity. Further, it has been a practical problem that discrimination of hydrophobicity/hydrophilicity before and after exposure, namely a change in solubility is small. For example, for securing hydrophilicity after exposure, a recording layer is forced to become hydrophilic to a certain extent before exposure, and as a result, the ink receiving property of image portions of the resulting printing plate and strength in printing become insufficient.
On the other hand, as another heat mode positive type plate precursor, U.S. Pat. No. 3,574,657 and JP-A No. 50-113307 suggest a plate precursor having a constitution in which a hydrophobic recording layer which can be removed by heat mode exposure is provided on a hydrophilic substrate. The principle of image recording of this plate precursor is based on the fact that the layer structure of a recording layer is destroyed by exposure, and as a result, the recording layer is removed in the exposure or printing processes, unlike the above-described plate production based on a change in the solubility and dispersibility of a recording layer in a heat mode solution type positive plate precursor. However, in a heat mode exposure removal type positive plate precursor, complete removal of a hydrophobic recording layer is difficult, and manifestation of sufficient hydrophilicity of non-image portions is difficult. On the other hand, there is the dilemma that when the thickness of a recording layer is decreased to enhance the removability thereof, a deterioration is caused in the strength of the image portions, and a printing plate having only a low printing resistance is obtained.